1. Field of the Invention
The present invention relates generally to an automatic gain control (AGC) circuit and control method therefor. More particularly, the invention relates to an AGC circuit and control method therefor amplifying a received multiplexed signal of a Code Division Multiple Access (CDMA) system or so forth.
2. Description of the Related Art
The conventional AGC circuit is designed to increase a gain of an amplifier circuit when an input voltage is lower than a reference voltage and to decrease the gain when the input voltage is higher than the reference voltage for the purpose of obtaining an output of a constant level and less distortion. Accordingly, the reference voltage is fixed at a predetermined value.
In case of a Time Division Multiple Access (TDMA) system and a Frequency Division Multiple Access (FDMA) system, not so significant problem may be encountered even in the system, in which the reference voltage is fixed.
The reason is that, TDMA and FDMA systems are systems, which permit one receiver to occupy a frequency axis in time sequence. In these system, a relationship between an average value of a reception voltage and a maximum value thereof can be checked in advance by a parameter (such as in a roll off ratio in a Quadrature Phase Shift Keying (QPSK)) of a modulation system.
With the reference voltage thus determined, a signal having substantially constant level and less distortion can be obtained as the output of the AGC circuit. Accordingly, full scale of an analog-to-digital converter (A/D converter) provided on an output side of the AGC circuit can be used effectively.
Next, CDMA system will be discussed briefly. FIG. 11 is a frequency spectrum chart showing a structure of a transmission signal in CDMA system. In FIG. 11, a vertical axis represents an output level (dB) and a horizontal axis represents a frequency (Hz).
In FIG. 11, CDMA system is a system, in which signals from a plurality of stations, e.g. 3 stations of U1 to U3, are transmitted in multiplexed fashion in the same frequency band f. Namely, the signals of a plurality of stations are transmitted in multiplexed fashion at the same time and at the same frequency.
In such modulation system according to CDMA, in which a plurality of receivers commonly occupy the frequency axis, a relationship between the average value and the maximum value of the reception voltage is variable depending upon number of receivers at instantaneous timing to frequently cause difficulty in prevention of occurrence of distortion of the input signal of the A/D converter.
When a number of the receivers receiving the signals at the same timing is large, peak factor (crest factor) of the reception waveform is obviously increased. On the other hand, in order to prevent increasing of the peak factor, a reception S/N can be degraded by setting the reference voltage sufficiently low with respect to the full scale of the A/D converter.
On the other hand, one example of the AGC circuit variable of the reference voltage has been disclosed in Japanese Unexamined Patent Publication No. Heisei 7-226725 which will be hereinafter referred to as first prior art. The disclosed first prior art has been worked out with giving importance for amplification of a small input voltage.
FIG. 12 is a block diagram showing a construction of the conventional AGC circuit disclosed in the first prior art. Referring to FIG. 12, the AGC circuit converts a received orthogonal frequency division multiplexed signal S50 into a baseband by a frequency converting portion 101 and amplifies a baseband signal S51 after conversion by an AGC amplifier 102, and transmits the baseband signal S52 amplified by the AGC amplifier 102 to mixers 106 and 107 by dividing into two paths by a divider 103.
The mixers 106 and 107 mix the respectively input baseband signals S53 and S54 with signals S55 and S56 output from a phase converter 105 and having phase difference of 900 and output signals as first and second baseband signals S57 and S58.
The first baseband signal S57 is converted into a digital signal S59 by an A/D converter 110 via a low-pass filter 108. Similarly, the second baseband signal S58 is converted into a digital signal S60 by an A/D converter 111 via a low-pass filter 109.
Outputs S59 and S60 of the A/D converters 110 and 111 are detected voltages by a rectifier circuit 114, and a voltage S62 depending upon a detected value S61 is generated by a control voltage generator 116.
On the other hand, a voltage generator 120 generates a constant voltage S63. A voltage adder 118 adds the constant voltage S63 to a voltage S62 generated by the control voltage generator 116 to control a gain of the AGC amplifier 102 by a resultant voltage S64.
The voltage S63 to be added is a voltage for controlling the gain of the AGC amplifier 102 so that input waveform to the A/D converters 110 and 111 exceeds allowable inputs of the A/D converters 110 and 111.
When input waveform to the A/D converters 110 and 111 exceeds allowable inputs of the A/D converters 110 and 111, the quantization error for the small amplitude portion of the input waveform to the A/D converters 110 and 111 would be reduced.
The large amplitude portion of the input waveforms to the A/D converters 110 and 111 are clipped exceeding an allowable voltage range to cause influence of distortion or so forth. What is demodulated by a demodulation device is an Orthogonal Frequency Division Multiplexing (OFDM) wave. Considering OFDM wave, occurrence probability of the large amplitude portion is quite small not to cause significant problem in comparison with improvement effect.
FIG. 13 is a frequency spectrum chart of OFDM wave. As shown in FIG. 13, OFDM wave causes mutual interference of a signal at a center frequency f1 and a signal at a center frequency f2 within a frequency region f12, for example, and causes mutual interference of a signal at a center frequency f2 and a signal at a center frequency f3 within a frequency region f23.
However, mutually interfering signals are only two signals and is completely different from the case of CDMA. In the present invention generally handles CDMA mutually interfering three or more signals.
Namely, when the AGC circuit as disclosed in the first prior art is employed in CDMA, the large amplitude portion is clipped exceeding the allowable voltage range of the A/D converter to encounter a problem to cause large distortion or so forth.
Other example of this kind of AGC circuit has been disclosed in Japanese Unexamined Patent Publication No. Heisei 8-331192 (hereinafter referred to as second prior art). The disclosed invention is directed to a Quadrature Amplitude Modulation (QAM) demodulation device which demodulates a modulated wave signal by performing phase correction based on accurate synchronization detection by simple construction and simple calculation.
The disclosed AGC circuit performs gain adjustment by extracting upper M in number of values of the amplitude values of symbol signals. However, an object of the invention disclosed in the second prior art is to demodulate the modulated wave signal by performing phase correction on the basis of accurate synchronization detection, which is far different from an object of the present invention.
Accordingly, even in the second prior art, there has not been disclosed means for preventing occurrence of distortion in the input signal to the A/D converter in a CDMA demodulation device, as a task of the present invention.
Therefore, an object of the present invention is to provide an AGC circuit and control method therefor which can prevent occurrence of distortion of an input signal of an A/D converter and whereby to enable use of full scale of the A/D converter even upon demodulation of signals of modulation system, in which a frequency axis is occupied by a plurality of receivers, such as CDMA.
According to the first aspect of the present invention, an AGC circuit comprises:
amplifying means for amplifying an input signal;
control means comparing an amplitude of the input signal and a first reference value and controlling a gain of the amplifying means depending upon a result of comparison; and
first reference value control means for detecting a maximum value of the amplitude of the input signal amplified by the amplifying means and controlling the first reference value depending upon the maximum value.
According to the second aspect of the present invention, a control method of an AGC circuit comprises:
a first process of amplifying an input signal, comparing an amplitude of the amplified input signal with a first reference value and controlling an amplitude of the input signal depending upon the result of comparison; and
a second process of detecting a maximum value of an amplitude of the amplified input signal with a predetermined period and controlling the first reference value depending upon the maximum value.
With the present invention set forth above, the first reference value, to be compared with the input signal is varied depending upon the maximum value of the amplitude of the input signal to control the gain of the amplifying means.
By the construction set forth above, the AGC circuit is capable of prevention of occurrence of distortion of an input signal of an A/D converter and enables use of full scale of the A/D converter even in demodulation of a signal of a modulation system, in which a frequency axis is commonly occupied by a plurality of receivers, such as CDMA.